The present invention relates to the blocking of plastic lens blanks, that is, their support during the prescription grinding thereof. Such plastic lenses are conventionally provided in blank form having one surface thereof of a standard convex configuration and the other surface thereof adapted to be ground to the desired concave curvature. In order to adequately hold or position the blank with regard to the grinding machine, the blank is blocked, that is, a low melting point alloy is applied to the rear surface of the blank and molded in such a shape that the boss or block thus formed can be held or chucked to a portion of a grinding device so as to positively support the lens blank during the grinding operation.
In order to accomplish conventional blocking of a lens blank, it is known to initially place a flexible pad having an adhesive face in contact with the convex surface of the blank and thereafter mold, as by casting, melted blocking alloy directly to the rear surface of such pad. The pad is preferably of substantial thickness, on the order of 10 mils or more, and is formed of a heat insulative material such that when the alloy is applied directly thereto, the pad will at least to some extent serve to insulate the blank from the adverse effect of such heat, i.e., distortion of blank.
It has been found, however, that in many cases with the plastic lens blanks under consideration, the heat insulative effect of such pad is not adequate and accordingly some distortion of the lens blank takes place, which distortion impairs the optical accuracy of the lens being produced. Furthermore, the blank cannot be re-blocked should corrective grinding be required, because the ground lens is now so thin that re-blocking will invariably result in severe distortion. Any attempt to increase the thickness of the pad so as to correspondingly increase the heat insulative effect would be undesirable since such pads undesirably enable increased relative movement to occur between the block and the lens because of the inherent flexible nature of the materials utilized in forming the pad, which movement adversely affects the accuracy of the grinding operation. Such materials normally include natural or synthetic polymeric rubberlike materials such as polyvinyl chloride sheets and the like. Ideally, then, the pad should be extremely thin to reduce flexibility and formed of a highly insulative material so as to prevent lens distortion caused by the heat of the blocking material applied thereto. Unfortunately, such an ideal pad construction is not available.
A further drawback of known blocking procedures is that the necessary bulk to form the block portion of the blocked lens has been found, by reason of its relatively high weight, to contribute to undesirable distortion of the lens.
It is accordingly a primary object of the present invention to provide a lens blocking procedure which avoids the above indicated shortcomings of the prior art yet enables plastic lenses to be blocked and ground in an efficient, straight-forward manner, while at the same time utilizing conventional and existing blocking and grinding equipment.
A further object of the present invention is the provision of a process for enabling plastic lens blanks to be re-blocked if re-grinding is necessary.
Still another object of the present invention is the provision of a stiff, lightweight block component adapted to prevent heat distortion of an underlying lens blank when blocking material is applied directly to the block component and to further essentially fully support such lens blank during grinding.
These and other objects of the present invention are accomplished by the provision of a disc-like member having a concave surface shaped to closely conform to the convex surface of a standard plastic lens blank and having a hub upstanding from the rear surface thereof. Such member is adhesively connected to the lens blank by a very thin adhesive layer and thereafter low temperature melting point blocking material applied directly thereto and molded thereon so as to form an integrally formed block on the rear surface thereof. The member is of stiff, lightweight material having a melting or distortion point substantially higher than the melting point of the low temperature melting point blocking materials such that the member absorbs the heat from the blocking material so as to in effect insulate the lens blank and thus prevent heat distortion thereof.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.